Many washing machine designs include a rotating tub that holds clothing or other articles for washing. The tub also holds fluid, typically water and detergent, that is used in the washing process. During the washing process the tub rotates at various speeds depending upon the operating mode of the washing machine. For example, during an agitation phase the drum may rotate at a comparatively low speed and reverse rotational direction frequently. During a spin cycle, the tub typically rotates at a much higher rate to drain excess water from the tub.
As the rotational rate of the tub increases, the centripetal acceleration of the tub urges the load in the tub against a wall of the tub. In one operating mode, the mass of the load in the tub distributes in a substantially uniform manner around the tub. When the load is uniformly balanced, the tub can rotate at various operational speeds without generating undue vibration that can cause the washing machine to move or damage components in the washing machine. However, when a portion of the mass of the load is distributed in an uneven manner in the tub, the washing machine can exhibit unwanted vibration and “walk” or move in response to vibrational forces generated by the rotating tub. The uneven distribution of the load is referred to interchangeably as an “out of balance” or “imbalanced” load. The magnitude of the vibrational forces generated as the tub rotates with an out of balance load increases as the rate of rotation increases, so most of the unwanted vibration occurs when the tub rotates at a high rate of speed, such as during a spin cycle.
Existing techniques are known to identify an out of balance load in a tub that is rotating at comparatively low rates of rotation based on a measured torque of a motor that rotates the tub. However, as the rotational rate of the tub increases, the tub begins to move laterally within the washing machine in addition to rotating. Washing machines typically include a suspension to accommodate and dampen the lateral movement of the tub. When the tub undergoes lateral movement, the methods that measure torque to identify an out of balance load become inaccurate.
One method for identifying an imbalanced load at high speeds is described in U.S. Pat. No. 6,393,918. The '918 patent describes a method for comparing a measured electrical power used to accelerate a tub in a washing machine to a “standard” acceleration power level expected for operation of the washing machine. Challenges remain in determining what the “standard” power level for a washing machine should be, and in determining out of balance loads when a washing machine rotates a tub at a substantially constant rotational speed. Improved methods of identifying out of balance loads in washing machines operating at high rotational speeds would be beneficial.